1. Field of the Invention
The present invention relates to a novel organic nonlinear optical material and a nonlinear optical element having this organic nonlinear optical material which generates short wavelength light by conversion of a laser beam into a second harmonic wave.
2. Description of the Related Art
Light communication and light information processing are expected as basic technologies for the development of information in future. It is essential for these technologies to establish means for properly controlling the wavelength, the amplitude, and phases of laser beams, in other words harmonic generation, light switching, and light mixing are important aspects of nonlinear optical material. The utilization of the nonlinear optical effect is quite advantageous in controlling the properties of light at fast speeds.
Conventionally, inorganic ferroelectric crystals, for example bulk crystals made of LiNbO.sub.3 and KTP, are used for the conversion of light of wave lengths of YAG lasers and dye lasers. However, these inorganic materials cannot sufficiently yield the desired nonlinear optical effect. Because of this reason, they are merely used for high-output lasers, but they are used in to low-output semiconductor lasers which have been developed in recent years.
On the other hand, recently, a variety of organic compounds have been found, which have a far greater nonlinear optical constant than that of inorganic crystals and excellent durability against optical damage. It is expected that the utilization of these organic nonlinear optical materials will lead to elements which are capable of generating a sufficient second harmonic wave for low-output semiconductor lasers.
There are a number of technical papers which discuss organic nonlinear optical materials including the following: "Nonlinear Optical Properties of Organic and Polymeric Materials", by D. J. Williams et al., published by American Chemical Society, in 1983, and "Nonlinear Optical properties of Organic Molecules and Crystals", by D. S. Chelma and J. Zyss, published by Academic Press, in 1987, for example.
The feature of the molecular structure of such organic nonlinear optical materials is that an electron-releasing group and an electron-withdrawing group are attached to a .pi.-electron system like the benzene ring at opposite positions from each other.
However, the dipole moment in the ground state inevitably increases in these organic nonlinear optical materials having the above-mentioned molecular structure. In a solid made of molecules having a large dipole moment, energetic stability is obtained when the molecules are aligned with their dipole moments in opposite directions to each other, and thus the molecules are likely to be crystallized in the form of centrosymmetry. With regard to the centrosymmetric crystals, second harmonic generation is inhibited in principle. The nonlinear characteristic is promoted in such molecules which have wide spread .pi.-electron systems. 0n the other hand, the absorption band of the molecules superposes the wavelength of the fundamental wave or the second harmonic wave and retards effective second harmonic generation.
Furthermore, as in the case of using an inorganic compound, if a nonlinear optical element is manufactured by using a bulk crystal made of organic nonlinear optical material, some problems are encounted as described below. First, in order to produce bulk crystals made of organic nonlinear optical materials, those processes rarely applicable to organic materials must be executed, which include growth, cutting, and polishing of single-crystals. Crystals of organic compounds are in general molecular crystals containing weakly bonded molecules, in other words, have weaker mechanical strength than those of inorganic crystals, and thus much difficulty is involved in the execution of the above-mentioned processes.
Furthermore, when operating the wavelength converting element composed of bulk crystals, it is essential to achieve phase matching by compensating for the difference of phase velocity between the fundamental wave and the harmonic wave by means of the double refraction of crystals. However, even when using such crystals which are capable of most efficiently utilizing the .beta..sub.333 component which is presumably the largest nonlinear polarizability tensor component in terms of molecular level of organic compound, since the directions of the electric field vectors of the fundamental wave and second-harmonic wave are identical to each other, phase matching can hardly be attained.